LEDs are a form of semiconductor material that convert electrical energy into optical energy. In semiconductor LEDs, light is typically generated through recombination of electrons, originating from an n-type doped semiconductor layer, and holes originating from a p-type doped semiconductor layer. In some infra-red emitting semiconductor materials light can be generated by electron intersub-band transitions rather than electron hole transitions.
A major challenge in the field is to extract as much of the emitted light as possible from the semiconductor material into the surrounding medium, usually air. This is hindered by total internal reflection at the surfaces of the semiconductor.
On traditional cuboid shaped LED chips, the average path length for light rays within the semiconductor is long, and the average number of reflections of an emitted light ray at semiconductor surfaces is high, prior to escape. Long path lengths and reflections at metal coated semiconductor surfaces may lead to absorption losses.
A number of approaches have been applied to increase the amount of useful light from LEDs. These include the use of external reflecting mirrors and chip shaping. With all approaches there are also some associated drawbacks.
A further approach to achieving high extraction efficiency (EE) and maximum useable light from a chip is to provide an array of “micro-LEDs” (μLEDs), as such devices keep the average path length within the device short. Arrangements are described in U.S. Pat. No. 6,410,940 and U.S. Pat. No. 6,410,942.
U.S. Pat. No. 7,518,149 describes a μLED that is an integrated diode structure in a mesa, in which the mesa shape and the light-emitting region are chosen for efficiency. A μLED may include, on a substrate and a semiconductor layer, a mesa, a light emitting layer, and an electrical contact. The μLEDs in this device may have improved EE because of their shape. Light is generated within the mesa, which is shaped to enhance the escape probability of the light. In particular, improved EEs may be achieved with a near parabolic mesa that has a high aspect ratio. The top of the mesa is truncated above the light-emitted layer (LEL), providing a flat surface for the electronic contact on the top of the semiconductor mesa. It has been found that the efficiency is improved, provided the top contact has a good reflectivity value.
The result of this structure is that the μLED output light is quasi-collimated, making the μLED a type of semiconductor light source that can be positioned somewhere between a Laser (where the light is coherent and directional) and a standard LED (where the light is scattered in all directions). These μLED devices allow for quasi collimated beams to be produced with a focus of, for example, less than 30°. This can be compared to standard unfocussed LEDs which produce beam angles in excess of 100°. The key benefits of being able to focus LED light is that less light is wasted because it is possible to get most or all the light generated by the LED into the space where it is needed, i.e. virtually all of the light generated is used, and little or none is wasted. This potentially results less power used by LED devices, which can lead to longer battery life in battery powered LED devices, and easier miniaturisation of devices as the light can be directed to where it is needed with the use of additional complicated optics.
LED arrays have a number of applications including LED printing, heads-up displays, active matrix displays systems and signage, amongst others. They are distinct from standard LEDs in their ability to allow for individual emitters be controlled and switched independently. For wide format printing it may be required to provide a linear array light source of say 35 cm or more wide, which are typically made up of a number of linear array modules. The linear array modules may be about 0.2 mm to 20 mm long and are abutted to form a longer linear array.
Modular LED arrays are disclosed, for example, in U.S. Pat. No. 6,683,421.
A problem encountered with available LED arrays, is that an array of individual LEDs on a chip may be closely spaced, but because of the packaging and housing surrounding such an array of LEDs, when two arrays (or chips) are abutted, the spacing between adjacent LEDs on the abutted chips can be quite large, much larger than the spacing between LEDs on the same chip. Similarly, high density square may use 5×5 diode arrays. These arrays may be arranged in groups to provide larger area high intensity arrays, for example as described in the above-mentioned U.S. Pat. No. 6,683,421. In another example, multiple LED arrays may be linearly arranged to provide a wide band of illumination.
In a typical square 5×5 LED array, for example, each LED die may be 1 mm2, so that the array may be about 7 mm×7 mm. However, the arrays are fabricated on a substrate, which has a 1 mm to 2 mm edge that surrounds each individual LED die. The die may be hermetically sealed within the package, which requires a minimum wall thickness around the LEDs to provide a good seal. When these LED arrays are abutted, there is a gap or spacing between adjacent groups of LEDs, which may be 2 mm to 4 mm. Thus, there is a limit on the pitch between LEDs on the arrays to maintain the pitch across neighbouring arrays due to packaging constraints. This can mean that there is uniform intensity along the length or width of the each module, but there is a dip in intensity/irradiance in the region where each module abuts, which tends to cause a banding effect in the substrate being cured.
U.S. Pat. No. 6,450,664 shows a modular LED array assembly, which provides a denser arrangement of LEDs near ends of the assembly to provide a uniform irradiance profile with a sharper edge, i.e. approaching a rectangular or “top hat” function. U.S. Pat. No. 6,380,962 to Miyazaki provides an arrangement to provide an irradiance profile with a sharper edge using wider light source near ends of a linear light source. However, the problem of providing a more uniform irradiance profile where two modules abut is not addressed. In fact a sharp profile may exacerbate edge effects in modular arrays, i.e. creating a more marked discontinuity or dip in irradiance caused by the spacing where two modules abut, depending on the size of the gap or separation between LED elements due to the mechanical housing.
In another example, disclosed in U.S. Pat. No. 7,175,712, LEDs are arranged in staggered rows, and LED arrays are also staggered to provide a more uniform irradiance. However, because of the thickness of the substrate and packaging surrounding the array, this arrangement also does not overcome a discontinuity in irradiance around edges of the arrays, or where two arrays abut.
U.S. Pat. No. 6,515,309 presents a method where grooves are cut below the top surface of the LED material such that a neighbouring chip may be closely aligned and packaging issues due to chipping are reduced. In U.S. Pat. No. 6,515,309, the angled cuts are formed between the plane of the light emitting layer and the bottom of the chip (ie from top to bottom).
WO 2010130051 provides a method for fabricating monolithic LED arrays with high resolution. It aims to replicate an addressable array of LED emitters that is continuous, i.e. contains no dark area. It is not possible simply to contact the individual emitters due to constraints on the interconnects. In WO 2101130051, a constant array is replicated using two rows of emitters that are offset. This results in two complimentary rows which alternate between an emitting and a non-emitting area of the same size. In order to provide a constant line of light (i.e. a ID array), the emitters are moved and timing of LED illumination is controlled. WO 2101130051 also outlines a control scheme and optical arrangement for use with the above arrays.
Arrays fabricated on individual standard LED chips cannot, in general, reach resolutions less than 200 μm. This is due to the limit on the size of the individual chips. Therefore to produce LED arrays of high density/resolution, monolithic array chips must be used. A similar problem of packing density and intensity occurs for monolithic arrays. Although the emitters can be very closely packaged when on the same chip, when multiple chips are abutted to form longer arrays the packaging density is reduced.
With available LED arrays, for high speed printing applications using very short exposure times of the substrate to be cured, an array formed from individually packaged LED die, or a single row of LED die, may not provide sufficient intensity or resolution. It is then necessary to provide a higher density array of emitters to provide a line, a band, multiple lines, or multiple bands of illumination of higher intensity. Therefore, higher density arrays allow for faster printing or photo-curing and higher resolution printing as well as enabling a number of new applications.